This invention relates to camera lens and flash accessories, employing polarized filters, for controlling lightness ratios, or contrast, between two or more areas of a photograph.
The invention comprises camera accessories with which a polarized filter can be held and rotated in front of a camera lens or flash unit while a second polarized filter is moved linearly over part or all of the front of the lens or flash unit.
The invention offers practical assistance to photographers faced with three basic problems:
(a) With distant objects, such as landscapes and sunsets, one area of the scene often is so much brighter than the other that both cannot be properly recorded on film.
(b) With near objects, such as flowers and birds, sunlight often causes harsh shadows; the lack of sunlight often results in flat, featureless pictures.
(c) With near pictures, including flowers, movement of the subject often causes blurred pictures.
The problem with distant pictures is discussed in many books and articles. For example, the following comments are found on page 204 of "The New Joy of Photography" by the editors of Kodak: "The light in a landscape varies tremendously in brightness. The supremely sensitive human eye adjusts instantaneously for these extreme conditions. But photographic film is sometimes incapable of recording both shadows and highlights". This problem has been partially solved in four ways:
(a) Kodak's editors suggest that "you may have to expose for one and sacrifice the other. More often you will want to determine an intermediate exposure by taking a close-up reading of a middle value in the scene or by metering both light and dark areas and splitting the difference." This is good advice in the absence of other methods. The fact is, however, that sacrificing one area or averaging frequently results in land areas that are too dark or sky areas that are too light. Averaged photographs can be printed darker or lighter, but this is just another way of sacrificing one or the other.
(b) It is common practice to use a polarized filter to darken blue skies. This is an effective technique, but the darkening effect decreases rapidly as the camera is turned away from a 90.degree. angle between sun and object. It is ineffective in the cases of pictures toward or away from the sun and in the absence of blue skies.
(c) Darkroom techniques known as "dodging and burning" can be used, during printing, to decrease the apparent contrast between dark and light areas of a photograph. many photographers, however, do not have easy access to darkrooms and do not want to pay for custom printing services. In any event, these techniques do not provide negatives for reproduction unless the added expense of producing new negatives from prints is accepted.
(d) The sky in a landscape picture can be darkened with a "graduated filter", meaning that half of the filter is colored to diminish transmitted light and the other half is clear. The "Cokin" filter system offers a number of colors, including two neutral gray filters of different densities. These filters are effective but they have certain disadvantages: they are expensive, they cannot be cut to fit the contour between land and sky, they are difficult to see within a camera's viewing system, they offer limited density variations, and they must be replaced for changes in density.
The problems with near pictures also are discussed in available literature. For example, in the aforementioned book on pages 140 and 142, Kodak's editors discuss the problems of contrast between areas of highlights and shadows. On page 194, they discuss the problem of blurred pictures due to subject movement. In the first case they suggest the use of light reflectors; in the second, they suggest the use of wind baffles. In both cases they suggest the use of flash units for fill-lighting or main-lighting, as well as for "freezing" moving object. In some cases, they suggest the use of diffusing materials, such as tissue or cloth, to reduce flash output.
These are useful techniques for selecting lens apertures, shutter speeds and flash distances. Frequently, however, the flash-to- subject distances are not convenient. Without assistants or special equipment, the photographer is handicapped unless the flash unit can be located at or near the camera. Some flexibility is provided by flash units with variable outputs but they are expensive and offer a limited range of f-stops. Expedient diffusers are useful, but they must be used based on trial-and-error tests and they do not add much flexibility.